Chemiosmotic Coupling
February 21, 2003
Bryant Miles
The four major complexes of the electron transport chain operate independently in the inner mitochondrial
membrane. Each of the complexes is an aggregate of proteins that are held firmly together by
noncovalent forces. There is no experimental evidence that the four complexes associate with one another
in the membrane. Each of these complexes has its own rate of lateral diffusion through the bilipid
membrane which shows that the complexes do not move together. Kinetic studies of reconstituted
electron transport systems support the theory of four independent complexes.
The model for the electron transport system is shown above. Four independent complexes are transferring
electrons through the mobile electron carriers of CoQ and cytochrome c. CoQH2 is produced by both
complex I and complex II and delivers the electron to complex III via the Q-cycle. Complex III reduces
cytochrome c which is a water soluble electron carrier located in the intermembrane space of the
mitochondria. The reduced cytochrome c carries the electrons to complex IV which transfers the
electrons to molecular oxygen. The process of electron transfer is coupled to transporting protons from
the matrix to the intermembrane space of the mitochondria. This generates a chemical potential and an
electrostatic potential. This potential energy is used to drive the synthesis of ATP.
Complex I:
Complex II:
Complex III:
Complex IV:
NADH + 5H+N + Q NAD+ + QH2 + 4H+P
FADH2 + Q FAD + QH2
QH2 + 2H+N + 2Cyt c (Fe3+) 2Cyt c (Fe2+) + Q + 4H+P
4Cyt c (Fe2+) + 8H+N + O24Cyt c (Fe3+) + 4H+P + 2H2O
Overall reaction beginning with NADH:
NADH + 11H+N + ½O2 NAD+ + 10H+P + H2O (10H+P/2e-)
Overall reaction beginning with FADH2:
FADH2 + 6H+N + ½O2 FAD + 6H+P + H2O (6H+P/2e-)
NADH + H+ + ½O2 NAD+ + H2O
NAD+ + 2e- + H+ NADH
Eo’ = −0.315 V
+
Eo’ = +0.816 V
½O2+ 2e + 2H H2O
∆Eo’ = +0.816 V – (−0.315 V) = 1.136 V
∆Go’ = −nF∆Eo’ = −219 kJ/mol
FADH2+ ½O2 FAD + H2O
FAD + 2e- + H+ FADH2
½O2+ 2e- + 2H+ H2O
∆Eo’ = +0.816 V – (0 V) = 0.816 V
∆Go’ = −nF∆Eo’ = −157.5 kJ/mol
Eo’ ≈ 0.000 V
Eo’ = +0.816 V
Most of the free energy from the transfer of electrons from NADH or FADH2 to molecular oxygen has
been used to pump electrons out of the matrix across the inner mitochondrial membrane into the
intermembrane space. Recall from BICH410 the electrochemical energy inherent in this difference of
proton concentration.
∆G = RTln(C2/C1) + ZF∆ψ
Where C2/C1 is the concentration ratio for the ion being transported, Z is the absolute value of the ions
electrical charge which is 1 for a proton. And ∆ψ is the transmembrane electrical potential measured in
volts.
When one talks about hydrogen ion concentrations, one usually talks in terms of pH = −log[H+].
The log function (base 10) and the natural log function (base e) are related by the following relationship.
Ln(x) = 2.303Log(x)
∆G = 2.303RTlog([H+P] /[H+N]) + ZF∆ψ
∆G = 2.303RT(log [H+P] - log[H+N]) + ZF∆ψ
∆G = 2.303RT(-pHP + pHN) + ZF∆ψ
∆G = 2.303RT(pHN -pHP) + ZF∆ψ
let ∆pH = (pHN -pHP)
∆G = 2.303RT∆pH + ZF∆ψ
Ζ = 1; F = 96.4 kJ/molxV
∆G = 2.303RT∆pH + 96.4 kJ/molxV ∆ψ
This free energy is called proton-motive force.
Actively respiring mitochondria have: ∆pH = 0.75 pH units; ∆ψ ≈ 0.17 V; T=298 oK, R = 8.3145 kJ/molxoK
∆G = 2.303(2.478kJ/mol)(0.75) + 96.4 kJ/molxV (0.17 V) = 20.6 kJ/mol
This is the free energy required to pump a proton across the inner mitochondrial membrane under
respiring conditions.
To pump 10 protons across the membrane ∆G = 206 kJ
For NADH (10H+P/2e-) ∆Go’ = −219 kJ
All but 13 kJ/mol of the free energy is used to actively transport the protons across the membrane.
Similarly for FADH2 (6H+P/2e-) ∆Go’ = −157.5 kJ/mol
To pump six protons across the membrane ∆G = 6 X 20.6 kJ = 124 kJ.
All but 33.5 kJ of the free energy is used to actively transport the protons across the membrane.
ATP Synthase
How is the concentration gradient of protons across the inner mitochondrial membrane used to generate
ATP? The proton motive force drives the synthesis of ATP as protons flow back from the intermembrane
space to the matrix of the mitochondria. The protons are channeled through an enzyme call ATP synthase
which catalyzes the following reaction.
ADP + Pi + nH+P ATP + H2O + nH+N.
Within the mitochondrial membrane, there is a complex of proteins that carries out ATP synthesis called
ATP synthase or F1F0-ATPase (named for the reverse reaction it catalyzes). ATP synthase is composed
of two principle complexes, the F1 unit which catalyzes the synthesis of ATP. This F1 unit is associated
with an integral membrane protein aggregate, the F0 unit. The F0 unit forms a transmembrane pore
through which protons are channeled to drive ATP synthesis. The F1 unit is composed of five polypeptide
chains α,β,γ,δ, and ε which a stoichiometry of α3β3γδε. The α and β subunits are homologous to each
other , each subunit contains an ATP binding site. The catalytic sites are located in the 3 β subunits.
The Structure of ATP synthase.
Shown in Figure (b) is a side view of the F1 unit. It contains 3 α subunits and 3 β subunits arranged like
the segments of an orange. The γ subunit forms a shaft. (c) is the top view of the F1 unit. The single γ
subunit associates primarily with one of the αβ pairs forcing each of the β subunits into a different
conformation. One β subunit has an ADP bound, the next β-subunit contains ATP the next is empty.
This difference in nucleotide binding among the three b subunits is critical to the mechanism of the
complex.
Shown to the far left is the side
view of the F1F0 structure. The
F1 complex is in purple and grey
and the F0 complex is shown is
shades of yellow and red.
To the immediate left is ATP
synthase viewed from the P-face
towards the N face.
The F0 subunit is composed of
three subunits denoted a, b and c
in stoichiometry of ab2c10-12. The
c subunits are the alpha helices
that span the membrane with a
small extending out into the
matrix side of the membrane.
The c subunits are arranged into
two concentric circles with a 55
Å diameter. The ring of csubunits forms a rotor that turns with respect to the a-subunit.
The 2 b-subunits of F0 complex associate firmly with the α and β subunits of F1 holding them fixed
relative to the membrane. In the membrane embedded cylinder (subunit c) of F0 is attached the shaft
composed of the γ and ε subunits of F1. As protons flow through the membrane from the P side to the N
side through the F0 channel the c subunits turn which in turn turns the embedded shaft which rotates
causing the β-subunits to change conformation as the γ subunit turns.
In the presence of a proton gradient, ATP synthase catalyzes the following reaction:
ADP +Pi + E [ExADPxPi] [ExATP] E + ATP
In the absence of a proton gradient, there is no net synthesis of ATP, but ATP synthase catalyzes the
exchange of the hydroxyl groups of inorganic phosphate with the aqueous solvent as shown by the
incorporation of O18 water into phosphate shown to
NH
the left.
2
N
O
-
O
P
O
O
+
OH
-
N
O
P
O
-
O
P
O-
N
O
This shows that ATP synthesis does not require the
input of energy, but the release of the newly
synthesized ATP does require energy. The movement
of protons through the F0 channel causes the γ subunit
to rotate which drives a conformational change in the
structure of the β-subunit resulting in the binding of
substrates (ADP and Pi) and the release of the product
ATP.
N
O
O-
H
H
OH
OH
H
H
H+
NH2
N
H2O
O
O
-
O
P
O
N
O
P
O
-
O
-
O
P
N
O
N
O
O-
H
H
OH
OH
H
H
E
H2O
18
NH2
N
H+
O
-
O
O
P
O
OH
18-
+
-
O
P
O
-
O
N
O
P
O
N
N
O
O-
H
H
OH
OH
H
H
The F1 complex of ATP Synthase has three interacting
conformations and three conformationally distinct active
sites. The Open conformation is inactive and has a low
affinity for ligands. The L form has a loose affinity for ADP
and Pi and also inactive. The tight conformation is active and
has a high affinity for ADP and Pi. The synthesis of ATP is
initiated by the binding of ADP and Pi to an open L site. In
the next step, an proton driven conformational change
converts the L conformation into the T conformation and
simultaneously converts the O form to the L form and the
T form to the O form. In the third step ATP is synthesized at
The T site and ATP is released from the O site.
This cycle continues over and over.